Date of Graduation

5-2017

Document Type

Thesis

Degree Name

Bachelor of Science in Civil Engineering

Degree Level

Undergraduate

Department

Civil Engineering

Advisor/Mentor

Hale, W. Micah

Committee Member/Reader

Prinz, Gary

Committee Member/Second Reader

Bernhardt, Michelle L.

Abstract

Prestressing of concrete is the introduction of permanent internal stresses in a structure or system in order to improve its performance. Concrete is strong in compression but weak in tension. The tensile strength of concrete is approximately 10% of the concrete’s compressive strength. Prestressing strands helps counteract this by introducing compressive stress in the area that will experience tensile stress because of the service load. In precast prestressed concrete girders, strands are placed in the bottom flange of the girder. These strands are tensioned to approximately 75% of their ultimate tensile capacity. After placing the concrete and after the required compressive strength has been achieved, the strands are cut and the tension forces transfer from the strands to the concrete. This creates a large compressive stress in the bottom flange. The eccentricity of the pretensioned strands in the prestressed concrete girders creates a bending moment that causes the girder to deflect upward, and this is called camber. This camber is reduced by the downward deflection of the girder due to the girder self-weight.

Camber in prestressed concrete girders is effected by several factors, such as the girder’s cross sectional properties, concrete material properties, strand stress, ambient temperature, and relative humidity. Some methods of predicting camber use the initial camber that occurs immediately after cutting the strands to predict the camber at the time of girder erection. There are many sources of errors in predicting camber in a concrete girder including the differences in the actual and the design value of concrete properties and of strand stress.

In this study, the difference between the measured and the predicted initial camber will be investigated on six AASHTO Type VI girders. The initial camber was predicted using the simple elastic analysis. The measured initial camber was then compared with the design camber. The difference between using the gross section properties and the transformed section properties to predict camber was quantified. Actual concrete properties including compressive strength, elastic modulus and unit weight were used to assess the current design method. Camber obtained from the actual, measured concrete properties will be called the predicted camber in this study. The effect of using the actual and the design elastic shortening losses on the estimation of the initial camber was also quantified.

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